There are a variety of reasons to stretch films. PCT WO 00/29197 discloses a method of biaxially stretching a polymeric film. The method may be used to impart mechanical characteristics to products such as film backing.
Stretching may enhance physical properties of crystalline plastic films. U.S. Pat. No. 2,998,772 discloses a machine for stretching film that includes circular discs that grasp edge portions of a film and stretch the film transverse to a machine direction of the film.
FIG. 1 illustrates a conventional tenter drawing process that stretches continuously fed films transversely to the direction of film travel. The film is gripped at both edges 2 by some gripping means, typically by tenter clips. The tenter clips are connected to tenter chains that ride along linearly diverging tenter tracks or rails. This arrangement propels the film forward in a machine direction of film travel and stretches the film. Thus an initial shape 4 in the film may be stretched to the shape 6.
Tenter apparatus are described in U.S. Pat. Nos. 2,618,012, 3,502,766, 3,890,421, 4,330,499; 4,525,317 and 4,853,602. Conventional tenters suffer many drawbacks. The angle of divergence in conventional tenters is typically small, usually less than 10 degrees. Boundary trajectories return to a parallel, or nearly parallel, state prior to quenching the polymeric film and slitting. Referring to FIG. 2, the unstretched portion 4 of the film shown in FIG. 1 may have dimensions T, W and L. After the film is stretched by a factor of lambda (7), the dimensions of that portion of film have changed to those shown on portion 6. This is not uniaxial stretch as described in greater detail below.
As used herein, the ratio of the final T′ to initial thickness of the film T (see FIG. 10) may be defined as the normal direction draw ratio (NDDR). The machine direction draw ratio (MDDR) may be defined as the length of a portion of the film after stretching divided by the initial length of that portion. For illustrative purposes only, see Y′/Y in FIG. 11. The transverse direction draw ratio (TDDR) may be defined as the width of a portion of the film after stretching divided by the initial width of that portion. For illustrative purposes only, see X0/X in FIG. 9.
The NDDR is roughly the reciprocal of the TDDR in a conventional tenter, while the MDDR is essentially unchanged. This asymmetry in MDDR and NDDR draw causes differences in the various molecular, mechanical and optical properties of the film above and beyond the differences in properties between these directions and the stretch direction (TD). Illustrative examples of such properties include the crystal orientation and morphology, thermal and hygroscopic expansions, the small strain anisotropic mechanical compliances, tear resistance, creep resistance, shrinkage, the refractive indices and absorption coefficients at various wavelengths.
U.S. Pat. No. 4,862,564 discloses an apparatus for stretching a thermoplastic material web. The device includes an exponential or other curvilinear stretching profile. The apparatus provides a constant rate of stretch to the web, as opposed to the sharp peak and varying rate of stretch provided with conventional straight course tenter apparatus.
Uniaxially drawn films have superior performance to simply monoaxially drawn films. For example, uniaxially drawn films are more easily fibrillated or torn along the stretch direction (TD). In optical applications, matching the MD and ND indices of refraction is often advantageous. For example, U.S. Pat. Nos. 5,882,774; 5,962,114; and 5,965,247 (Jonza, et. al.) disclose materials with matched indexes of refraction for improved off-normal angle performance in brightness enhancement applications of multilayer reflective polarizers.
FIG. 3 illustrates a known batch technique for stretching a multilayer film suitable for use as a component in an optical device such as a polarizer. The flat, initial film 3 is stretched uniaxially in the direction of the arrows. The central portion necks down so that two edges of the film are no longer parallel after the stretching process. Much of the stretched film 5 is unusable as an optical component. Only a relatively small central portion 9 of the film is suitable for use in an optical component such as a polarizer. The yield and usable part size from this process are small.
Japanese Unexamined Patent Publication Hei 5-11114 teaches that compensation films with matched MD and ND indices of refraction allow wider viewing angles in liquid crystalline displays.
A conventional method for attempting to make a uniaxially drawn film is to use a length orienter (L.O.) that draws the film longitudinally in MD across at least one span between rollers of differing speed. The MDDR imparted along this span or draw gap is essentially the ratio of the speed of the downstream roll to the upstream roll. Because the film freely spans the rollers without edge constraints, the film can neck down in width as well as thin in caliper as it draws. Thus the TDDR can be reduced substantially below unity and can possibly be made to equal the NDDR. The method is fraught with difficulties and limitations. One disadvantage is the limitation on part size. An initial web of given width is reduced in width by a factor of the square root of the reciprocal of MDDR. Thus a final film made with an L.O. has a substantially reduced width. When contrasted to a film made by a tenter, which increases the width by roughly the TDDR (excluding edge losses from gripping), the L.O. under uniaxial conditions reduces the possible part size substantially.
Stretching longitudinally tends to amplify machine direction propagated caliper imperfections such as die lines. In order to achieve a high degree of uniaxial character, the L.O. needs a long span relative to the film initial width. Practically, this requires a large device and long film spans that may be hard to control.
Japanese Unexamined Patent Publication Hei 6-34815 points out another limitation of making films for optical applications over rollers. This document points out that rollers can scratch or otherwise damage the surface of the film. Films with delicate coatings or with soft skin layers could be easily deleteriously impacted.
In Japanese Unexamined Patent Publication Hei-150115, the effective initial width is reduced by introducing MD oriented slits into the film in a periodic fashion. This method even more severely limits the available part width.
There have been many attempts to draw films in a uniaxial fashion. Japanese Unexamined Patent Publication Nos. Hei 5-288931, 5-288932, 6-27321 and 6-34815 (H. Field, et. al.) describe methods where film is fed into clips whose gripping surfaces form an out-of-plane waveform. Since the actual contour length along MD of the film is much longer than the in-plane projection of that contour length along MD of the tenter, the actual rate of film fed in is higher than its planar projection. The film is initially fed in a similar out-of-plane waveform (e.g. it is corrugated). The method makes use of the MD tension that develops during draw to take up the slack of the corrugation and flatten the final film. In a variation, the film is drawn normally and then placed in the waveform clips. Heat treatment under tension after draw and the resulting shrinkage forces are then relied on to flatten the web. The method is described in conjunction with polysulfone films at low levels of overfeed (under 20%). The method is likely limited by process issues such as the draw ratio range required and heat transfer. Many useful uniaxially oriented films require draw ratios in excess of 4. These in turn would require overfeeds in excess of 100%, resulting in deep out-of-plane folds that would be difficult to heat uniformly. For example, the heat transfer to the tops and bottoms of the folds could be much higher than in the center plane due to the closer proximity to the heating plenums. This would tend to limit line speeds. Such large folds could also collapse and stick to each other as the web strength weakened in the pre-heat needed to effect draw, thereby causing the method to fail. At low levels of overfeed, the method reports good flattening across the film. As the boundary waveform became deeper, it is believed that the yield and quality of the final film would be adversely impacted.
Japanese Unexamined Patent Publications, Hei 5-241021, 6-51116 and 6-51119 disclose clip gripping surfaces remaining in-plane during draw. The film is fed into the clips at an out-of-plane angle while the clips are moving around an out-of-plane radius. The out-of-plane radius creates a temporary increase in the separation between the individual clips. After rounding the curve, the clip gripping surfaces return in-plane, the clips remain separated but more closely spaced, and corrugated portions of the film provide extra slack lie between the clips. The method relies on the tension during draw to flatten the film in-plane. The method may suffer the disadvantages of large corrugations for high draw ratio conditions. Additionally, since the clips remain separated prior to draw, the edges of the film forming the initial corrugations are unsupported. As the drawing proceeds and stresses build, these unsupported edges begin to pull inwards towards the film centerline. Eventually large scallops form between the clips. The scallops not only make the edges unusable, but also create large caliper variations through the film. This adversely impacts the yield and quality of the final film.
Japanese Unexamined Patent Publications, Hei 5-11113 discloses decoupling the MD line speed from the instantaneous film MD velocity by making the process partly discontinuous in mass flow. Transversely oriented slits are introduced into the web. These allow central portions of the otherwise continuous film to pull away from each other, allegedly allowing more substantially uniaxially drawn material in these portions. This method puts severe limits on usable part size and yield.
U.S. Pat. No. 4,349,500 (Yazawa, et. al.) discloses a film fed between two rotating disks or wheels. The film is gripped by two continuous belts. The film and the disks all lie in the same plane. The film stretches transversely between the counter rotating disks as its edges follow the diverging circumferential edges of the disks. The divergence angle of the draw becomes large, and the MD velocity of the film slows by the cosine of this divergence angle. The belt speed remains constant. In this manner, the output velocity is reduced from the input velocity of the film. The film is released from its gripping belts and the film is taken up at the slower MD velocity.
The method discloses the adjustment of the separation distance between the centers of rotation of the disks and the size of the disks. One disadvantage of this method, discussed in U.S. Pat. No. 5,826,314, is the difficulty of maintaining good gripping of the film with the belt system. This would be particularly challenging in the stretching of films that develop high levels of drawing stress, e.g. polyesters drawn near their glass transition temperatures. It is believed that many materials used in this process would acquire a wrinkle or a non-uniaxially drawn permanent set using this method. For example, polyesters monoaxially drawn near their glass transitions while holding their MD lengths fixed may wrinkle rather than snap back in-plane when the final width is reduced in a succeeding step towards that anticipated for the substantially uniaxial case. Wrinkling also can occur when the MD reduction is applied too late in the TD drawing step.
Swenson U.S. Pat. No. 5,043,036 describes a canted wheel film drawing apparatus. Here the disks are no longer in-plane with the film and thus the sheet is stretched between out-of-plane boundary trajectories defined by the circumferences of the canted wheels. The method is described as a means of stretching films comprising elastomeric layers. As pointed out in U.S. Pat. No. 3,807,004, due to the developing MD tension along the progress of the draw, stretching between such out-of-plane curved surfaces causes the film surface to become saddle shaped. The central portion of the film straightens out as it is not directly held, as is the film at the boundary trajectories, and thus it draws along a different path than the edges. This non-uniform drawing can result in significant caliper and property variations across the web, and is a major disadvantage for drawing films along boundary trajectories that move out-of-plane.
U.S. Pat. No. 3,807,004 described a variety of methods for partially dealing with the saddle formation. Profiling the initial film thickness or temperature distribution is suggested as a means to uniform caliper, although property variations due to different drawing histories would remain. Alternatively, a support device could force the film in the central portion to conform to the curved out-of-plane trajectory. Friction and concomitant damage to the film surface might be reduced by various methods including an air cushion. Saddling also manifests in various operations with the aforementioned disk orienter as described in U.S. Pat. No. 4,434,128. A convex guide surface is used to counter the saddling. Damage to the film surface from the application of such methods is another disadvantage to the method. In particular, films used in optical applications are particularly sensitive to surface defects as may be caused by scuffing and other contact-related defects.